The present disclosure relates to a method of performing a D2D discovery and a terminal using the method and, more particularly to, a D2D discovery performing method of comparing a congestion level of a discovery slot selected according to a predetermined or preset condition with a threshold or a congestion level of a current discovery slot and determining a discovery slot to transmit a discovery message and a transmission period of the discovery message according to a result of the comparison, and a method thereof.
According to the development of wireless communication technologies, a Device-to-Device Communication (hereinafter, referred to as D2D communication) technology in which adjacent terminals perform direct communication without the use of a cellular network is spotlighted.
The D2D communication largely includes a D2D discovery for discovering an adjacent terminal or an application program of the terminal and D2D direct communication in which terminals actually exchange data.
A wireless communication system supporting the D2D communication should allocate some or all of the wireless communication resources as discovery resources for the D2D discovery. D2D terminals broadcast discovery messages by using predetermined discovery slots among the allocated discovery resources through the wireless communication system. At this time, the terminals select predetermined discovery slots stochastically or according to a particular pattern and individually transmit the D2D discovery messages. Accordingly, as the number of terminals increases, a congestion level of limited discovery resources increases and collisions between discovery messages are frequently generated.
In order to minimize the loss, a method by which the terminal can select an efficient discovery slot, which is suitable for the discovery, is required.
In a CSMA/CA method corresponding to a random access method used for technologies such as IEEE 802.11 wireless LAN, IEEE 802.15 WPAN, and the like, when a transmission terminal transmits a packet but does not successfully receive an ACK packet of the corresponding packet, it is considered as a collision of the transmitted packet and the packet is retransmitted after a backoff for a predetermined time.
The CSMA/CA method is mainly used for one-to-one communication and is used when the ACK packet of the transmitted packet can be received. However, the D2D discovery does not guarantee the reception of the ACK packet, so that the CSMA/CA method cannot be used for selecting the discovery slot.
According to the conventional D2D discovery method, the terminal may continuously monitor discovery slots and then select a discovery slot having weak energy by a discovery signal of another terminal. When a discovery message collision is detected in the selected discovery slot, the terminal may select another discovery slot having weak energy and broadcast a discovery message.
However, in the conventional discovery slot selection methods, the terminal monitors neighboring discovery slots one by one and thus it is inefficient. Further, when the number of terminals using the network increases, the conventional discovery slot selection methods basically cannot control the congestion.